The present invention, in some embodiments thereof, relates to antimicrobial agents, and more particularly, but not exclusively, to non-resistance inducing antimicrobial conjugates which are effective also against resistant bacteria, and to uses thereof in treating infections.
Early advancements in the field of antibiotics had transformed medical care and dramatically reduced illness and death from infectious diseases. However, over the decades, almost all the prominent infection-causing bacterial strains have developed resistance to antibiotics. Among the different classes of clinically important antibiotics that largely suffered from the resistance problem during the last few decades, is the aminoglycoside class of drugs. These antibiotics have broad-spectrum of activity against both Gram-negative and Gram-positive bacteria by selectively targeting bacterial protein synthesis machinery, and have been used for over fifty years. Such a prolonged clinical and veterinary use of currently available aminoglycosides has resulted in effective selection of resistance, which severely limits their usefulness.
Due to the limitations associated with the use of classical antibiotics, extensive studies have been focused on finding novel, efficient and non-resistance inducing antimicrobial/antibacterial agents.
The most prevalent mechanism in clinical isolates of resistant bacteria is the bacterial acquisition of aminoglycoside-modifying enzymes, which modify the antibiotics by N-acetyltransferase (AAC), 0-phosphotransferase (APH), and O-nucleotidylyltransferase (ANT) activities. Among these enzymes families, aminoglycoside 3′-phosphotransferases (APH(3′)s), of which seven isozymes are known, are widely represented. These enzymes catalyze phosphorylation at the 3′-OH to of both neomycin and kanamycin classes of aminoglycosides, rendering the resulting phosphorylated products inactive.
Although most of these enzymes are typically monofunctional enzymes, the recent emergence of genes encoding bifunctional aminoglycoside-modifying enzymes is another complication relevant to the clinical use of aminoglycosides. Among these enzymes, the bifunctional AAC(6′)/APH(2″) enzyme has been detected in Enterococcus, Staphylococcus, and Streptococcus isolates, including the methicillin-resistant Staphylococcus aureus (MRSA), and has been the most extensively investigated, due to the large number of clinically important aminoglycosides that are susceptible for modification with this enzyme.
To tackle the problem of bacterial resistance caused by enzymatic modification, many analogs of aminoglycosides have been synthesized by direct chemical modification of existing aminoglycoside drugs [1, 2]. Earlier investigations in this direction have yielded several semi-synthetic drugs such as amikacin, dibekacin, and arbekacin [1, 3]. However, new resistance to these drugs has emerged soon after their introduction to the clinic [4, 5].
One strategy that has been pursued in recent years to overcome bacterial resistance to aminoglycoside drugs employs a combination of two different drugs in one molecule [6]. With this strategy, each drug moiety is designed to bind independently to two different biological targets and synchronously accumulate at both target sites. Such dual action drugs, also referred to as hybrid drugs or conjugate drugs, offer the possibility to overcome current resistance. In addition, these conjugate drugs may reduce the appearance of new resistant strains [7].
Several applications of this approach have been reported [8-10]. The dual action compounds, combining fluoroquinolone (enrofloxacin or norfloxacin) and cephalosporin (cefamandole) moieties with an amide linkage, were found potent against Enterobacter species [9]. Fluoroquinolone-anilinouracil conjugates linked via their secondary amino groups have also been synthesized [10]. A series of oxazolidinone-quinolone conjugate structures, which simultaneously act on two different cellular functions, DNA replication and protein synthesis, have been reported [7, 11]. Lead compounds of this series exhibited a balanced dual mode of action and overcome the majority of known resistance mechanisms to quinolones and linezolid in clinically relevant Gram-positive pathogens.
Investigations towards glycopeptide/beta-lactam heterodimers were reported, employing vancomycin and cephalosporin synthons which were chemically linked to yield heterodimer antibiotics [12].
U.S. Pat. No. 7,635,685 (see also [13]) teaches modifications of aminoglycoside neomycin B (NeoB) by linking a variety of sugars at C5″-OH group via glycosidic linkage, which results with a class of pseudo-pentasaccharides that exhibited similar or better antibacterial activities to that of the parent NeoB against selected bacterial strains. However, while the specificity constant values (kcat/Km) of these derivatives with the aminoglycoside resistance enzyme APH(3′)-IIIa were in general lower than that of NeoB, the compounds exhibited inhibition values about 10-fold lower than that of NeoB, suggesting that several different conformations of the designed structures can bind the APH(3′)-IIIa productively and lead to the enzyme-catalyzed phosphoryl transfer process.
Several other derivatives of non-sugar modifications of NeoB at the C5″-position were reported to exhibit enhanced antibacterial activity compared to the parent NeoB [14], however these derivatives also exhibited substrate promiscuity with respect to APH(3′)-IIIa.
Additional background art includes a review of recent patent literature concerning heterodimers antibiotics [15], WO 2003/044034 and U.S. Patent Application having publication No. 2008300199.